1. Field of the Invention
This invention relates to an image forming apparatus such as a printer, a copier, a facsimile, or a multifunction apparatus which has some of these function, which forms an image using dot images.
2. Discussion of the Background
A known process for forming apparatus an image by dot images involves using a printer controller where the vector image data sent from an external apparatus, for example a host computer, is extended and stored as image memory data, or an original image data read by scanner and is stored as image memory data.
Then, these stored image data are read out by predetermined timing, and gamma correction is carried out for each dot. This gamma correction is carried out by changing writing density of dot or changing size of dot. By this gamma correction, the image data is corrected for a nonlinear characteristic of printer, and then, after halftoning process, for example a multilevel dithering process, the image data is sent to a printer engine.
In the printer engine, the image data from the printer controller is corrected for a variation of density, and printing out is executed according to corrected image data. In other words, a laser beam is modulated, power modulation or width modulation, according to the corrected image data, and dot images are formed on a recording paper by electrophotography.
However, in this process, an image forming apparatus is in the background, because the gamma correction is executed before the halftoning process, a number of levels of halftone of the printed image decreases. For example, when the number of levels of halftone of the image data which are read out from the image memory is 256 levels, the number of levels of halftone of a printed image is about 170 levels, so that the number of levels of halftone increases 2/3 times.